Electrochemical process

ABSTRACT

There is disclosed an electrochemical process wherein an electrolyte is passed through one electrode compartment of an electrochemical cell, said one electrode compartment comprising a particulate electrode and being separated from a second electrode compartment of said electrochemical cell containing a counter-electrode by a separator having an ion-permeable wall inclined away from the vertical and towards the particulate electrode. The distribution of the particles of said particulate electrode in the electrolyte during the process is controlled in a manner such that there is formed within said one electrode compartment a first region which is adjacent to the ion-permeable wall and within which first region substantially all the particles are, for a large proportion of the time they spend in said first region, separated from each other, and a second region which is spaced from the ion-permeable wall and within which second region substantially all the particles are, for a large proportion of the time they spend in said second region, in contact with other particles, and substantially all the particles are circulated between said first and second regions.

The invention has particular application to the electrodeposition ofmetals in which metal is electrodeposited onto the particles of aparticulate cathode which are removed from the cathode compartment.

BACKGROUND OF THE INVENTION

This invention relates to electrochemical processes and apparatus and isconcerned with electrochemical processes and electrochemical cellsemploying particulate electrodes.

In general, electrochemical processes may be considered as being eithercathodic processes or anodic processes depending on the electrode atwhich the technically important reaction occurs. Most cathodic processesinvolve either metal electrodeposition or electrolytic reduction of aconstituent of the electrolyte in the presence of hydrogen formed at thecathode; in the former class of cathodic process are electroplating,electrorefining and electrowinning and in the latter class are thereduction of organic compounds and the production of caustic soda. Mostanodic processes involve either the discharge of anions from solution atan essentially stable anode or the dissolution of the anode itself; inthe former class of anodic process are processes for the production ofchlorine and oxygen and in the latter class are processes for recoveringvaluable metal from scrap and the refining or purification of metals.Further details of industrial electrochemical processes are given in thebook "Industrial Electrochemical Processes" edited by A. Kuhn andpublished by Elsevier in 1971.

The present invention is primarily, but not exclusively, concerned withcathodic processes, especially processes for electrowinning metals, andwith particulate electrodes and electrochemical cells incorporatingparticulate electrodes which can be used in such processes.Electrowinning generally involves the recovery of a metal by depositionof the metal from aqueous leach liquors obtained by leaching an ore orconcentrate with an aqueous electrolyte. Conventional electrowinningprocesses employ cells containing planar or grid-type anodes and planarcathodes. The anodes are generally insoluble and serve to conductelectricity to the electrolyte but in some instances the anode mayconsist of, for example, a corrodible matte anode. The nature of thecathode at the start of the process varies widely from process toprocess and may be, for example, a stainless steel, a titanium or analuminium electrode on to which a thin layer of relatively pure metal isthen deposited or a thin sheet of relatively pure metal (called astarting sheet and usually produced by deposition of the metal on toelectrodes such as those mentioned above). During the electrowinningprocesses the metal to be recovered is deposited on the cathode which ispermitted to grow to the desired thickness. The fully grown cathode isthen removed from the cell for further processing. The electrolyteusually consists of an aqueous solution of one or more salts of themetal which solution is formulated so as to promote electrodeposition ofthe metal on the cathode in such form and purity as is desired and atacceptable power efficiencies. The cathodic current density is limitedto relatively low values, e.g. 100-350 A/m² for copper electrowinning,by the mass transfer effects at the cathode. In practice, operatingconventional electrowinning cells above a certain critical currentdensity would yield unacceptably rough, and therefore impure, cathodicproducts. The value of this critical current density is limited by therate of the mass transfer processes transporting metal ions from thebulk of the electrolyte to the cathode and is a function of thefollowing variables:

1. Concentration of metal ions in the electrolyte.

2. Conductivity of the electrolyte.

3. Concentration overpotential.

4. Activation overpotential.

5. Presence of impurities and solids.

6. Presence of additives, such as levelling agents, brightness etc.

In recent years there have been described particulate electrodes whichcomprise a number of discrete particles consisting wholly or partiallyof electroconductive material and which, when the electrode is in use,are caused to move so as to be in intermittent contact either directlyor through the agency of intermediate particles with at least onecurrent conductor (which is often called the "current feeder" or "feederelectrode") by means of which an electric current is conducted to theparticles. The electrical conductivity of the current conductor isgenerally not less than 10⁴ ohm⁻ ¹ cm⁻ ¹.

Particulate electrodes have been developed in a number of differentforms. In one form, a mixture of the particles of the electrode and anelectrolyte is pumped through a portion of the cell which contains thecurrent conductor and in which the electrode reaction occurs, thenaround a circuit outside this portion of the cell, and is finallyreturned to the portion in which the electrode reaction occurs forfurther reaction. In another form, the particles of the electrode remainwithin the portion of the cell which contains the current conductor --while an electrolyte only is passed through this portion and then arounda circuit outside this portion. Included within this latter form areelectrodes which, in operation, comprise a bed of particles throughwhich there is an upward, evenly distributed, flow of the electrolyte;the particles become suspended in the electrolyte which flows at a ratesuch that the bed becomes expanded in volume, usually by more than 20and an generally by 40 to 50 percent. The pattern of flow of electrolyteis arranged to be substantially constant through a horizontalcross-section within the bed of particles in order to achievesubstantial uniformity of particle concentration in the horizontalplane. The terminology of fluidized beds has been applied to this formof particulate electrode and many of the properties of fluidized bedsare evident in the behaviour of these so-called "fluidized bedelectrodes". The high surface area of a fluidized bed electrode makespossible either the efficient electrolysis of dilute solutions or theuse of a high current per unit volume of cell and per unit volume ofelectrolyte; for example, in copper deposition current densities up to3000 A/m² and more have been used experimentally. Particulate electrodeshave been the subject of much research recently and examples of theirformation, including fluidized bed electrodes, and their use in variouselectrochemical processes are disclosed in, for example, British Pat.Specification No. 1,194,181, U.S. Pat. Nos. 3,180,810, 3,527,617,3,551,207 and 3,703,446, French Pat. No. 1,500,269 and Canadian Pat. No.700,933.

In many electrochemical processes using particulate electrodes theelectrode reaction involves deposition of ions on to the particles ordissolution of the material of the particles. In such processes thedimensions of the particles change with time and there may be provisionfor removal and replenishment of the particles. The choice of workingconditions in such processes may be influenced by:

1. the need to avoid agglomeration of the particles of the electrode,paticularly the particles of a cathode on which electrodeposition ofmetal is taking place and particularly at diaphragms interposed betweenthe anode and the cathode where agglomeration or plating has in the pastbeen found to occur frequently;

2. the need to avoid excessive electrodeposition of the product of acathode reaction on to the current conductor or agglomeration of theparticles on the current conductor; and

3. the need to ensure a satisfactory rate of progress of the electrodereaction.

4. the need to obtain acceptable power efficiencies. Whilst the tendencytowards agglomeration of particles may be reduced by increasing the rateof flow of electrolyte through the bed of particles, this increase inrate of flow in turn may reduce the rate of passage of charge from thecurrent conductor to the particles of the electrode but increases therate of passage of charge from the current conductor to the electrolyteand thus may increase the quantity of the product of the electrodereaction deposited at the current conductor.

It is an object of the present invention to provide an electrochemicalprocess employing a particulate electrode in which the disadvantages ofthe particulate electrodes referred to above are ameliorated.

It is a further object of the invention to provide an improved methodfor the electrodeposition of metals.

It is a still further object of the invention to provide a novelparticulate electrode, and a novel electrochemical cell incorporatingsaid particulate electrode, suitable for use in electrochemicalprocesses, including the electrowinning of metals such as copper, cobaltand nickel.

SUMMARY OF THE INVENTION

According to one aspect of the present invention there is provided, inan electrochemical process wherein an electrolyte is passed through oneelectrode compartment of an electrochemical cell, said one electrodecompartment comprising a particulate electrode and being separated froma second electrode compartment of said electrochemical cell containing acounter-electrode by a separator having an ion-permeable wall, theinprovement which comprises controlling the distribution of theparticles of said particulate electrode in the electrolyte during theprocess in a manner such that there is formed within said one electrodecompartment a first region which is adjacent to the ion-permeable walland within which first region substantially all the particles are, for alarge proportion of the time they spend in said first region, separatedfrom each other, and a second region which is spaced from theion-permeable wall and within which second region substantially all theparticles are, for a large proportion of the time they spend in saidsecond region, in contact with other particles, and circulatingsubstantially all the particles between said first and second regions.

According to a second aspect of the present invention there is providedan electrode system, suitable for use with a counter electrode toperform an electrochemical process in accordance with the invention,which electrode system comprises a particulate electrode, a currentconductor, a vessel containing said particulate electrode and currentconductor and having an ion-permeable wall at least a part of which isinclined towards the particulate electrode, and means for flowing afluid medium through said vessel in contact with said particulateelectrode.

According to a third aspect of the present invention there is providedan electrochemical cell which comprises at least one particulateelectrode system according to the second aspect of the invention and atleast one counter-electrode separated from said particulate electrode bysaid ion-permeable wall.

When the distribution of the particles is controlled in accordance withthe process of the invention, the density of particles adjacent to theion-permeable wall is low and there is formed at the back of said oneelectrode compartment, i.e. remote from said ion-permeable wall, aregion spaced from the ion-permeable wall in which the density ofparticles is high and approaches that of a static settled bed of theseparticles.

DESCRIPTION OF EMBODIMENTS OF THE INVENTION

The current conductor (or current feeder) is advantageously disposedwithin the second region, i.e. the high density region. Disposing thecurrent conductor in the high density region enables electrical chargeto be rapidly and efficiently conveyed amongst the particles of theparticulate electrode in the high density region with little or noelectrodeposition of metal taking place on the current conductor.Furthermore, with the low density region adjacent to the ion-permeablewall, which may be a fragile membrane or diaphragm, there can beexpected a lower risk of failure of the ion-permeable wall due toagglomeration and adherence of particles to the ion-permeable wall withsubsequent electrodeposition on or within the ion-permeable wall itself.

With the current conductor disposed in a high density region its exactlocation is far less critical than is the case, for example, with afluidized bed electrode. A current conductor in the form of a singleconductor rod located anywhere in the high density region is generallysufficient to ensure that electrical current is provided to all parts ofthis region. However, depending on the amount of current to be conveyedto the particles, it may be desirable to provide alternative designs ofcurrent conductor. A particularly advantageous design of currentconductor comprises a flat plate recessed into the wall of the electrodecompartment opposite the ion-permeable wall. Current is supplied to thiscurrent conductor by means of a conductor bar from the top of theelectrode compartment. It has been found that with a particulateelectrode consisting of copper particles the effective area of thecurrent conductor plate need only be about 5% of the area of the face ofthe particulate bed adjacent to the back wall of the compartment but itmay be larger, especially for particles consisting of material of lowerelectrical conductivity.

The desired distribution of the particles of said particulate electrodein the electrolyte may be controlled, at least in part, by appropriateflow of a fluid medium through the particulate electrode. Theelectrolyte which is passed through the electrode compartment containingthe particulate electrode can be used conveniently as the fluid medium.In this case, it is preferable if the electrolyte is caused to flowupwardly through the particulate electrode in such a manner as to giverise to circulation of the particles of the particulate electrodeupwardly adjacent the ion-permeable wall and downwardly at the back ofthe electrode compartment. In this way most of the particles arecirculated upwardly through a major proportion of the height of saidfirst region and downwardly through a major proportion of the height ofsaid second region. The desired electrolyte flow can be imposed byinclining the ion-permeable wall towards the particulate electrode asrequired by a particulate electrode system according to the secondaspect of the invention.

The angle of inclination of the ion-permeable wall to the vertical andtowards the particulate electrode will depend to some extent upon thedesign of the remainder of the particulate electrode system. The angleof inclination will lie within the range of from 1° to 45° from thevertical. It has been found that while some designs operate well at aninclination below 10°, e.g. from 3° to 6°, others operate satisfactorilyat greater inclinations, e.g. from 15° to 25°.

The distribution of the particles is controlled so that the overallvolume expansion, i.e. the volume occupied by the bed of particleswithin the electrode compartment which bed includes both high and lowdensity regions, during the process is less than 25% greater than thatof a static settled bed of the particles. Generally, the overall volumeexpansion will not exceed 20% and it is believed to be preferable forthe overall volume expansion to lie in the range of from 3% to 13%, forexample 8% to 12%.

The electrode compartment containing the particulate electrodeconveniently has a configuration in which its height and its width aresubstantially greater than its thickness, i.e. the distance between theion-permeable wall and the opposite wall thereto, i.e. the back wall.The height and width of the electrode compartment may each be of theorder of 50 to 100 centimeters or more, whereas the thickness of thecompartment is generally less than 10 centimeters, for example 5centimeters. The particulate electrode system has performed well whenthe electrode compartment is of truncated wedge-shaped form. In thisform of compartment, both the ion-permeable wall and the back wall ofthe compartment, are inclined to the vertical with the back wall havingthe greater inclination. It has also been found that compartments inwhich the back wall is parallel to the ion-permeable wall give goodresults. Electrode compartments comprising an upper, plane parallelconfiguration and a lower, wedge shaped configuration and compartmentscomprising an upper, wedge shaped configuration and a lower, planeparallel configuration have also performed well. Experiments withsubstantially wedge shaped compartments have indicated that thepreferred range of wedge taper angles is from 1:20 to 1:5, with the bestwedge taper angle being about 1:10 or less as the electrode compartmentheight is increased. (A wedge taper angle of 1:10 means that for every10 cm up the ion-permeable wall the back wall is spaced a further 1 cmfrom the ion-permeable wall.) However, the optimum angle depends on theheight of the particulate electrode during the process.

In one embodiment of the invention, the electrochemical cell may haveadded to it a second identical particulate electrode compartment on theopposite side of the compartment containing the counter-electrode. Inthis event, the compartment containing the counter-electrode wouldassume a wedge-shape to ensure an equal and directionally oppositeinclination for the added particulate electrode compartment. In afurther embodiment, the electrode compartment is repeated vertically,e.g. cathode compartments can be stacked vertically.

The fluid medium, which may be the electrolyte, is introduced to thecompartment containing the particulate electrode through a flowdistributor in the base of the compartment. Usually, the flowdistributor takes the form of a manifold feeding a plurality of inletpassages arranged along the width of the base of the electrodecompartment. Such flow distributors may further comprise predistributingsparge pipes beneath the inlet passages and wedge shaped portions in thebase of the electrode compartment immediately above the inlet passages.The inlet passages may comprise conical portions. In another form offlow distributor, it is proposed to locate an open helical coil alongthe width of the base of the electrode compartment. This provides a flowdistributor the aperture of which is easily adjusted and which is easilyclosed off, merely by closing the coil when the electrode system is notin operation. It may be advantageous to introduce additional fluidmedium for example, using fanning jets at one or more positions abovethe flow distributor and adjacent the ion permeable wall in order toassist in the control of particle distribution.

Undesirable disturbances in the flow pattern of particles in theelectrode compartment can be reduced by installing one or more flowdirectors within the electrode compartment. These flow directorsconveniently comprise one or more planar members disposed substantiallyvertically and are preferably parallel to, but may be normal to, theion-permeable wall. The flow directors need not divide the compartmentcompletely and thus may take a form of several members spaced from oneanother or may consist of mesh material which spans the whole or a partof the width of the compartment. Another way of modifying the flowcharacteristics in the electrode compartment is to employ a corrugatedion-permeable membrane. The corrugations of the membrane are arrangedvertically and function as short flow directors. Such a membrane may beemployed with or without other flow directors and has the furtheradvantages of greater rigidity and greater surface area than a planarmembrane.

In order to separate large particles from small particles during thecourse of the process, as may be desirable for example in anelectrowinning process, a sieve may be provided, for example, at the topof the bed of particles to catch the large particles. The sieve couldhave a mesh size suitable to return particles below a certain size tothe bed of particles and to retain the larger particles to be led offfrom the cell. Alternatively, use can be made of the hydraulicproperties of the bed of particles: a simple dip tube inserted primarilyinto the low density region with its end towards the bottom of theregion will conduct a stream of electrolyte bearing a number ofparticles well above the mean top level of the bed of particles in theelectrode compartment and this may be used to transfer particles fromthe electrode compartment. In another form the tube may be positioned ata slope, and may be branched to return finer particles directly to theelectrode compartment. Other ways of effecting the removal of particlesfrom the electrode compartment comprise installing a simple syphon inthe bed of particles or draining the particles from the base of the bedby a self-cleaning valve arrangement.

The structural members of the particulate electrode system andelectrochemical cells comprising such electrode systems are convenientlyconstructed from an electrically insulating, fluid-impermeable materialsuch as poly(vinyl chloride), a rubber or a poly(methyl methacrylate).Alternatively, these materials may be provided as coatings on otherconstructional materials such as steel. Other materials which may beused include concrete and glass fibre-reinforced plastics materials.

Materials which may be used for the ion-permeable wall include"Terylene" cloth, plastics material, such as poly(vinyl chloride), whichhave been rendered micro-porous, a polyester mat impregnated withphenolic rein, a porous ceramic material or an ion-exchange material.

To protect the ion-permeable wall from abrasion or other damage fromparticles, a holed non-conducting screen, such as a Terylene mesh, maybe placed adjacent the ion-permeable wall. This screen would serve toshield the ion-permeable wall from damage or deposition and it or theion-permeable wall may be removed for periodical cleaning andreplacement.

A wide range of electrically-conductive materials are available for theconstruction of the counter-electrode. This electrode may bedimensionally stable or may dissolve as the electrolytic processproceeds. Dimensionally stable anodes suitable for use in electrowinningprocesses conveniently take the form of a plate, mesh or grid oftitanium, activated on their surfaces by a coating of a noble metal or anoble metal oxide or a mixture of noble metal oxides and base metaloxides. Electrochemical cells according to the invention may employ asboth anode and cathode a particulate electrode system according to theinvention. However, when the electrochemical cell comprises oneparticulate electrode and one massive electrode, there will be asubstantial pressure difference across the ion-permeable wall betweenthese two electrodes. Unless the ion-permeable wall has substantialstrength the pressure difference may be sufficient to rupture it. It istherefore advantageous to provide means to support the ion-permeablewall against this pressure difference. Besides physically supporting themembrane, e.g. with a strong porous backing plate, one way to achievethis is to provide means to balance the operating pressures in the twocompartments. Thus, there may be provided in the compartment containingthe massive electrode a pressure regulator in the form of a flow impedercomprising a series of baffles, perforated plates or a packing. Whenelectrolyte flows through the compartment containing these flow impedersthe hydrostatic pressure is greater than that present in the absence offlow impeders and can be made substantially equal to that within theparticulate electrode compartment on the opposite side of the membrane.

A further advantage of installing such flow impeders is that thetendency for migration of electrolytes between compartments is reduced.This may make possible the replacement of the ion-permeable wall with acoarser and more robust, but more permeable, material, e.g. a filtercloth or a perforated plate.

The use of a particulate electrode system according to the presentinvention in an electrochemical cell makes it readily possible toprevent the escape of the "acid mist" which is liberated at theelectrolyte surface as the gas bubbles burst in the course of someelectrochemical processes. Thus a cover can be placed over the top ofthe electrode compartment and an outlet can be provided for collectionfor use of gases evolved at the electrode, while the acid mists may beseparated from the gases either inside or outside the cover.

The process, particulate electrode system and electrochemical cell ofthe present invention find particular application in theelectrodeposition of metals. More particularly, this invention isespecially applicable to the electrodeposition of metals such as gold,silver, copper, iron, the platinum group metals, cobalt, zinc, nickeland manganese from aqueous solutions of salts thereof. Suchelectrodeposition processes may be purification processes or metalelectrowinning processes. In such cases it is advantageous for theparticulate electrode system to constitute the cathode.

Thus, according to one embodiment of the present invention, there isprovided, in a process for electrowinning a metal wherein an electrolytecomprising an aqueous solution of one or more salts of said metal ispassed through a cathode compartment of an electrochemical cell, saidcathode compartment comprising a particulate cathode and being separatedfrom an anode compartment by a separator having an ion-permeable wall,the improvement which comprises introducing small particles into saidcathode compartment wherein they form part of the particulate cathode,extracting particles on which metal has been elecrodeposited from saidcathode compartment, controlling the distribution of the particles ofsaid particulate cathode in the electrolyte during the process in amanner such that there is formed within said cathode compartment a firstregion which is adjacent to the ion-permeable wall and within whichfirst region substantially all the particles are, for a large proportionof the time they spend in said first region, separated from each other,and a second region which is spaced from the ion-permeable wall andwithin which second region substantially all the particles are, for alarge proportion of the time they spend in said second region, incontact with other particles, and circulating substantially all theparticles between said first and second regions.

When the distribution of particles is controlled by appropriate flow ofthe electrolyte through the particulate cathode, in the manner describedabove, there is thought to be a diffuse transition region between saidfirst and second regions, in which transition region the density ofparticles graduates from that in the first region to that in the secondregion. Furthermore, there are normally present turbulent regions, bothat the top of the bed of particles and adjacent the flow distributor, inwhich turbulent regions the density of particles is not as great as insaid second region. Since electrodeposition in these turbulent regionsmay give rise to undesirable effects, it is advantageous to restrict theactive area of the ion permeable wall and/or the anode so as to renderthese turbulent regions electrochemically inactive. Preferably, theintroduction of the small particles and the extraction of the particlesin which metal has been electrodeposited is controlled so that apartfrom the small fluctuation produced by such introduction and extraction,the conditions within the electrode compartment do not changesignificantly during the process.

Electrochemical cells having a particulate cathode system in accordancewith the invention provide a very large ratio of cathode surface area tocell volume when compared with conventional electrowinning cells usingplanar electrodes and therefore for given cell current the effectivecathode current density is reduced compared to that in conventionalcells. For this reason even at relatively high cathode current densitiesof 1000 amps per square meter when expressed relative to theelectrochemically active area of the ion-permeable wall or to theprojected anode area the true cathode current density and therefore therate of mass transfer of metal ions to the cathodic particulate surfaceis very low. Therefore, even with low concentrations of metal ions inthe electrolyte it is possible to operate at high current efficiency andwithout polarization of the cathode. This means that when anelectrochemical cell according to the invention is used for recoveringmetal from electrolytes by electrowinning it affords a much moreflexible recovery process than is the case using conventionalelectrowinning cells which are often uneconomic, and therefore not used,for recovering metals from the more dilute solutions. It is thereforepossible using an electrochemical cell according to the invention todeplete the metal from normal concentrated electrolytes to much lowerlevels than is practicable with conventional elctrowinning cells whilestill maintaining an acceptably high current efficiency. This means thata greater quantity of metal can be extracted from a given volume andconcentration of electrolyte. Whereas, for example, it is commonpractice to deplete copper electrolyte from 40 gpl to 30 gpl copper in aconventional electrowinning plant, with a plant using electrochemicalcells according to the invention it is possible to increase the copperextraction without further loss of current efficiency or increase ofcathodic polarization, and to deplete the copper electrolyte from 40 gplcopper to, say, 5 gpl copper or to any selected concentration that wouldbe most economic for the particular process.

In an electrowinning process using dimensionally stable or inert anodes,the anode reaction with sulphate solution electrolytes reults in theliberation of oxygen, with chloride solution electrolytes it results inthe liberation of chlorine and with mixed sulphate/chloride solutionelectrolytes it results in the liberation of one or a mixture of thesegases depending on the relative concentration of the sulphate andchloride ions in the electrolyte. In all cases the liberation of thesegases and the subsequent bursting of the gas bubbles as they reach thesurface of the electrolyte causes minute quantities of electrolyte to bedispersed into the atmosphere causing the evolution of acid mist. Thepresence of acid mist is most undesirable because of its corrosiveproperties and in addition it is injurious to health. In conventionalelectrowinning cells various techniques are used to minimize theliberation of acid mist into the atmosphere. For example, plastic ballsare added to the surface of the electrolyte in the cell or organic mistsuppressants or foaming agents are added to the electrolyte. It is alsopossible to collect the anode gas in especially designed anode hoods orto cover the cell completely and to supply a slight suction to eitherthe anode hoods or cell cover to remove anode gas and acid mist. Allthese systems are costly, inconvenient, and not always fully effectivein supressing the liberation of acid mist into the atmosphere. With aparticulate electrode system, and an electrochemical cell according tothe invention, anode gas, and therefore acid mist collection, aresimplified, as has been mentioned hereinbefore. Thus, the collected gasmay be easily scrubbed free of acid mist at conveniently situatedscrubbers. In addition it is a simple matter to collect the anode gas inpure form without loss of gas or dilution of the gas by ingress of air.In the case of chloride solution electrolytes where highly toxicchlorine gas is liberated this is an especially important feature.Economic use of the collected anode gas can now readily be made.

With a process according to the present invention current densities ofup to 10,000 amps per square meter relative to the projected active areaof the anode have been successfully operated during copperelectrodeposition from aqueous solutions of copper salts. Even at thishigh current density, a strongly adherent, even deposition of copper onto the particles of the particulate electrode can be obtained. In suchprocesses depending on electrolyte concentration and current density,the potential difference between the anode and the current conductor maybe similar to or only slightly lower than those obtaining inconventional electro-winning cells.

The concentrations of the various ionic species present in theelectrolyte may be similar to those obtaining in conventionalelectrowinning processes. Thus the concentration of the species of metalion which is to be electrodeposited is usually around 50 gpl. Thesolution is normally acid and the cation present is often the sulphateion so that a concentration of between 50 gpl and 150 gpl of H₂ SO₄ isoften present. However, the properties of the particulate electrodesystem of the invention provide greater flexibility in the choice ofcatholyte compositions. Metal ions may be removed from the catholytedown to a concentration as low as a few parts per million (ppm).

Preferably, metal is electrodeposited on to particles composed of thesame metal. These metal particles will normally have sizes in the rangeof from 100 to 3000 μm. Preferably, the new particles introduced to thecathode compartment have sizes in a rather more restricted size range,for example from 200 to 1000 μm preferably from 200 to 700 μm. It ispossible to deposit one metal on to particles of another metal, e.g.cobalt or nickel can be deposited on to copper particles and such aprocedure may have application, for example, in the preparation ofalloys.

The current conductor, or current feeder, may be constructed of any goodelectrically-conductive material resistant to corrosion in theelectrolyte used. As stated above, this catholyte is often acId of lowpH and stainless steel is often a suitable material from which toprepare the current conductor under such conditions. Alternatively, thecurrent conductor may be constructed of the metal being electrodepositedso that any electrodeposition on to the conductor allows it to be soldas conventional cathode stock if electrodeposited metal builds up over along period of time.

BRIEF DESCRIPTION OF THE DRAWINGS

For a better understanding of the present invention, and to show moreclearly how the same may be carried into effect, reference will now bemade, by way of example, to the accompanying drawings, in which:

FIGS. 1 and 2 show vertical sections of two embodiments ofelectrochemical cells, the sections being normal to the width of thecells and showing the thickness of the cells;

FIG. 3 is a fragmentary section of part of a further embodiment of anelectrochemical cell;

FIG. 4 shows a section of the cell of FIG. 2 taken along the line IV--IVof FIG. 2 and looking towards the back wall of the cathode compartment;

FIG. 5 shows a perspective view of the cell of FIGS. 2 and 4;

FIG. 6 shows a perspective view of a further embodiment of anelectrochemical cell;

FIG. 7 shows a vertical section of the cell shown in FIG. 6, thissection being normal to the width of the cell and showing the thicknessof the cell; and

FIG. 8 shows schematically an electrodeposition process.

DETAILED DESCRIPTION OF THE DRAWINGS

Referring first to FIGS. 1 and 2, the electrochemical cells 1 eachcomprise a cathode compartment 2 and an anode compartment 3 separated byan ion-permeable wall 4. The anode compartment 3 is provided with ananolyte inlet 5 and an anolyte outlet 6. Baffles 7, which restrict theflow of anolyte E_(A) through the anode compartment, are included in theanode compartment. An anode comprising a platinum coated expandedtitanium mesh 8 is provided within the anode compartment. Current isconveyed to the anode by a conductor member 9 welded to the anode 8.Catholyte E_(c) can be introduced to the cathode compartment by means ofinlet conduits 10 at the base of the cathode compartment 2. These inletconduits 10 debouch into a chamber 11 below a flow distributor 12. Inthe embodiment shown in FIG. 2, the flow distributor 12 comprises aseries of conical passages 13. Catholyte can leave the compartment byoutlet conduits 14. Within the cathode compartment 2 there is provided abed of particles which comprises a region 15 of high density and aregion 16 of low density. Cathodic current conductor rods 17 extend intothe high density region 15.

FIG. 3 shows an alternative current conductor for the cathodecompartment of an electrochemical cell which is otherwise constructed asshown in FIG. 2. The current conductor comprises a plate 19 recessedinto the back wall of the cathode compartment. Current is conducted tothe plate 19 by a conductor member 20.

Referring now to FIG. 4, the disposition of the current conductor rods17 and the passages 13 within the flow distributor 12 of the cell shownin FIG. 2 can be seen more clearly. A conduit 21 is provided at the baseof the cathode compartment for the removal of particles containedtherein. In FIG. 5, external features of the electrochemical cell ofFIGS. 2 and 4 are shown more clearly. At the junction of the anodecompartment 3 and the cathode compartment 2 there are provided flanges23 and 22 respectively. Sandwiched between these two flanges is the ionpermeable wall 4. FIG. 5 also shows cathodic and anodic currentconducting members 9 and 24 respectively.

FIG. 6 shows in perspective view another embodiment of anelectrochemical cell according to the present invention. Anelectrochemical cell 101 comprises a cathode compartment 102 and ananode compartment 103. Catholyte is supplied to the cathode compartmentby a single inlet conduit 110 and leaves the compartment via a weir 125and outlet conduit 114. A single current conductor rod 117 is providedwithin the cathode compartment and the cell has a cover 151 throughwhich the current conductor rod 117 passes to a conductor member 124.

In FIG. 7 the cathode compartment 102 and anode compartment 103 are seento be separated by an ion permeable membrane 104. Anolyte is admitted tothe anode compartment by a single inlet conduit 105 and leaves thecompartment by a single outlet conduit 106. An anode 108 constructed ofsimilar materials to the anode 8 is recessed into the back wall of theanode compartment 103. Catholyte passes through a predistributing spargepipe 156 into a chamber 111 and then through a series of passages 113and a wedge shaped section 154 to the cathode compartment 102. In thecathode compartment is a bed of particles comprising a region of highdensity 115 and a region of a low density 116. A current conductor rod117 extends into the high density region 115.

FIG. 8 shows schematically an apparatus for carrying out anelectrowinning process in accordance with the invention. Catholyte isfed continuously to a cell 70 by a conduit 71 and leaves the cell byanother conduit 72. Catholyte is stored in a storage tank 73 and ispumped around the circuit including the tank and the cell by a pump 74.An isolating valve 75 and flow control valve 76 are provided in thiscircuit. A similar circuit comprising similar components 77 to 82 isprovided for the anolyte. A potential difference is applied to theelectrodes of the cell, current being provided by a d.c. rectifier 83.Particles are supplied from a feed stock tank 84 and are removedperiodically from the cell by the conduit 85.

During operation of the cell shown in FIGS. 2, 4 and 5, catholyte ispassed through the flow distributor 12 at a rate sufficient to lift theheaviest particles in the bed of particles. The catholyte tends totravel mainly in a first region 16 of the bed of particles adjacent tothe ion permeable membrane 4 creating a low particle density and highcatholyte flow rate therein. The particle density and catholyte flowrate in this first region 16 are low and high respectively relative to aregion 15 around the current conductor 17 which region 15 tends toremain less distributed by the more slowly upward flowing catholytetherein and thus to contain a greater density of particles than theregion 16 adjacent the ion permeable membrane 4. In the region 15surrounding the current conductor 17, the particles tend to slide orroll down the steeply inclined back wall of the compartment 2 generallyin contact with one another. The arrows shown in FIGS. 1 and 6 indicatethe flow patterns of movement of particles which are believed to takeplace. It appears that particles are carried from the vicinity of theflow distributor 12 by the catholyte up the low density region 16adjacent the ion permeable membrane 4 eventually to fall from thisregion under the force of gravity to be transferred to the high densityregion 15 surrounding the current conductor 17.

The flow of catholyte up through the high density region 15 is notsufficient to carry the particles with it, and the particles in thisregion thus tend to slide or roll downwards substantially parallel tothe back wall of the compartment to replace thode removed from thevicinity of the flow distributor 12. It is believed that not allparticles are carried by the catholyte flow to the top of the lowdensity region 16 of the bed of particles. It is thought that someparticles leave the low density region to join the high density region15 at various levels up the cell. It is also thought that particles mayleave the high density region 15 before reaching the flow distributor 12at the base of the compartment.

Flow of catholyte will in general be such that the overall volumeexpansion of the bed of particles within the cathode compartment doesnot exceed 20 %. The optimum overall volume expansion of the bed ofparticles for any particular case will, however, depend also on a numberof factors such as the density of the material of the particles, thedensity of the electrolyte, the flow rate of the electrolyte and so on.

In order to conduct an electrolytic process within the cell, anolyteE_(A) is passed through the anode compartment 3 so that the anode 8 isimmersed in anolyte and a potential difference is applied across thecurrent conductor 17 and anode 8. Metal is electrodeposited on to theparticles of the particulate cathode and these may be removedperiodically as they become larger. The size distribution of theparticles of the particulate electrode is maintained by the addition ofsmall feed particles to the particulate electrode. The operation of anelectrodeposition process according to the invention in anelectrochemical cell as shown in FIGS. 6 and 7 is similar to thatdescribed above with reference to the electrochemical cell shown inFIGS. 2, 4 and 5.

The particulate electrode system and electrochemical cell of the presentinvention give rise to a number of important advantages in theiroperation. As mentioned earlier the presence of a low density regionadjacent the ion permeable membrane prevents electrodeposition in thisarea. Furthermore the inclination of the ion permeable membrane awayfrom the vertical helps to prevent particles from settling on it and theaction of the electrolyte stream against it keeps it clean.Electrodeposition on to the current conductor is substantially preventedby the high density of particles surrounding it. However sufficientelectrolyte appears to permeate this high density region to preventconcentration polarization occurring there under typical operatingconditions. The flow of electrolyte through the particulate electrodecompartment and the movement of particles therein gives rise todesirable mixing effects. Furthermore, as mentioned above, the particlesin the high density region adjacent the current conductor are in closecontact, making for improved electrical conductor efficiency.

The invention is illustrated by the following Examples in which ExamplesI to VII are concerned with cathodic processes relating to theelectrodeposition of metals, and Example VIII is concerned with ananodic process relating to the dissolution of a metal from a matte.

EXAMPLE I

An electrochemical cell in accordance with the invention with an "IONAC"ion-exchange membrane constituting the ion-permeable wall was used toelectrowin copper from a solution of copper in sulphuric acid. Themembrane was inclined towards the particulate cathode and away from thevertical at an angle of 4°. The cell had a cathode compartment with awedge-shaped lower portion extending to a height of 19 cm and a parallelupper portion extending a further 20 cm. The wedge angle of the lowerportion was 51/2° (i.e. a wedge taper angle of 1:10). The width of thecell was 20 cm. The particulate cathode was formed from copper particlesranging in size from 300 μm to 800 μm, and a copper current conductor,or current feeder, was employed.

Catholyte, initially containing 50 grams per liter (gpl) of copper ascupric ions and 50 gpl of sulphuric acid, was passed at a rate of 350ml/cm² /min with respect to the median cross-sectional area through thecathode compartment of the cell. Meanwhile, sulphuric acid at aconcentration of 50 gpl was passed through the anode compartment.Electrodeposition took place at a current density of 2800 A/m² withrespect to the projected exposed effective area of the membrane.

Under these operating conditions, the concentration of copper insolution was reduced to 0.04 gpl. An overall current efficiency greaterthan 90 % was obtained, with no perceptible hydrogen gas evolution atthe cathode.

Other experiments were conducted at a current density of 7500 A/m² atwhich similar current efficiencies were measured.

EXAMPLE II

An electrochemical cell similar to that of Example I was used toelectrowin copper from a solution of copper in sulphuric acid in a batchrun lasting 16 hours.

The anolyte initially comprised 45 gpl of H₂ SO₄ while the catholytecontained 40 gpl copper and 56 gpl of H₂ SO₄. At the end of the batchrun the anolyte comprised 69 gpl of H₂ SO₄ with 10 ppm of copper, whilethe catholyte contained 0.01 gpl copper and 91 gpl of H₂ SO₄.

In this instance the membrane was inclined at 3° to the vertical and thecatholyte flow rate was initially 13 liters/minute increasing to 16liters/minute to maintain the overall volume expansion of the bed ofparticles between 8 and 12 % as the viscosity of the catholyte passingthrough the bed fell, with changing catholyte composition, from 1.035centipoises to 0.768 centipoises at a temperature of 40° ± 1°C.

During the experiment the current density averaged 1500 A/m² expressedrelative to the projected effective membrane area. The overall currentefficiency of the copper electrodeposition process was greater than 90%. The potential of the cathode was 220 mV with respect to a standardhydrogen electrode.

EXAMPLE III

The same cell as was used in Example II was used continuously in a batchrun lasting 125 hours. In this instance, the ion permeable membrane wasinclined at 4° to the vertical.

The anolyte initially comprised 50 gpl of H₂ SO₄ with 2 ppm of copperwhile the catholyte comprised 50 gpl of both H₂ SO₄ and copper. At theend of the batch run, the anolyte contained 103 gpl H₂ SO₄ with 58 ppmof copper while the catholyte contained 29 gpl of copper with 49 gpl ofH₂ SO₄.

The copper particles of the particulate cathode ranged in size from 212μm to 1200 μm and the bed of these particles was maintained at anoverall volume expansion of between 8% and 12% by a catholyte flowing ata rate increasing from 13.7 liters/minute to 17.0 liters/minute.

Copper production rate was 124 gms/hr, the fully grown particles beingremoved periodically. The average current density was 3000 A/m² (withrespect to the projected effective membrane area) while the currentefficiency of the electrodeposition process was around 95%. The cathodepotential against a standard hydrogen electrode was 210 mV.

EXAMPLE IV

A wedge shaped cell similar to that shown in FIGS. 2 to 5 of theaccompanying drawings was used to electrowin copper from an acid coppersulphate solution. The cell had a wedge angle of 5°, a width of 20 cmand a height of 50 cm. An "IONAC" ion exchange membrane, constitutingthe ion-permeable wall, was used to separate the anode and cathodecompartments. This membrane was inclined to the vertical at an angle of6°. The particulate cathode comprised copper particles whose sizes werein the range of from 250 μm to 900 μm.

Catholyte containing 3.26 gpl of copper and 22 gpl of sulphuric acid waspassed through the cathode compartment at a flow rate of 18 liters/min.to produce an overall volume expansion of the particulate cathode ofbetween 11 and 13 %. The catholyte contained a total impurity cationconcentration of about 10 gpl, the major impurities being Co, 1.88 gpl;Fe (total), 0.78 gpl; and Mg, 5.84 gpl. The temperature of the catholytewas maintained at 40°C ± 1°C.

Anolyte initially comprising 30 ppm copper and 54 gpl sulphuric acid waspassed through the anode compartment. Current was passed through thecell at an average current density of 2000 A/m² (with respect to theprojected active membrane area) until the final composition of thecatholyte was 0.24 gpl of copper and 25 gpl of sulphuric acid. At thispoint, the composition of the anolyte was 36 ppm of copper and 58 gpl ofsulphuric acid. The overall current efficiency of the copperelectrodeposition process was 99.6 %. The cathode potential with respectto the standard hydrogen electrode was 250 mV.

EXAMPLE V

A cell similar to that described in Example IV was used to reduce theconcentration of copper ions in a catholyte to a few parts per million.The particulate cathode had copper particles identical in size to thoseof Example IV and was operated at similar overall volume expansions.

Initially the catholyte contained 1.61 gpl of copper and 22.4 gpl ofsulphuric acid. The impurity cation concentration in the catholytetotalled 30 gpl, the major impurity being Co, 7.06 gpl; Fe (total), 2.94gpl; and Mg, 17.75 gpl. Initially the anolyte comprised less than 1 ppmof copper and 95 gpl of sulphuric acid.

The initial current density was about 2500 A/m² with respect to theprojected available membrane area but as the bulk copper concentrationin the catholyte decreased with electrodeposition of copper so thecurrent density was decreased to simulate a stepped, voltage controlledsituation. At the end of the experiment the concentration of copper ionsin the catholyte was 12 ppm, together with 26 gpl of sulphuric acid. Thecorresponding anolyte composition comprised 6 ppm of copper and 117 gplof sulphuric acid. The current efficiency of the overall process wasaround 60 % and this relatively low value was thought to be due to thepresence of Fe³ ⁺ ions. The current efficiency of copper electrowinningwas calculated to be around 90 %, the loss of around 10 % being due tohydrogen generation at the particulate cathode. During the experimentthe rate of flow of catholyte was increased from 19.1 liters/min. to 23liters/min. as the viscosity of the catholyte fell with changingcomposition. The potential of the cathode with respect to a standardhydrogen electrode varied between the limits of 260 and 223 mV.

EXAMPLE VI

A cell similar to that shown in FIGS. 6 and 7 of the accompanyingdrawings was used to electrowin cobalt from a cobalt sulphate solution.

The ion-permeable wall of the cell comprises a "DARAK 5000" porousmembrane, a material made by W. R. Grace & Co. The membrane was inclinedat 15° to the vertical. A stainless steel current conductor and copperparticles having sizes in the range of from 300 μm to 1200 μm were used.

In this experiment, both anolyte and catholyte had the same composition,this being initially 30 gpl of cobalt in a sulphate solution of pH 2.7.At an average imposed potential difference of 9.5 V, an average currentdensity of 2500 A/m², with respect to the active area of the membrane,was measured.

The rate of flow of catholyte through the cathode compartment was of theorder of 3.8 liters/min. resulting in an overall volume expansion of thebed of particles of around 10 %.

Final concentrations of ionic species within the electrolyte, i.e. bothanolyte and catholyte were 15 gpl of cobalt at pH 0.3.

EXAMPLE VII

A similar experiment to that of Example VI was conducted using an acidcopper sulphate solution as electrolyte. The composition of theelectrolyte was maintained at 50 gpl of copper and 100 gpl of H₂ SO₄.

Satisfactory electrodeposition of copper was obtained at a currentdensity of 8000 A/m², again with respect to the active area of themembrane.

Copper particles in the size range 420-1000 μm were used and asparticles were removed, feed particles ranging in size from 420 to 500μm were added.

EXAMPLE VIII

An electrochemical cell similar to that shown in FIGS. 2 to 5, but inwhich the particulate electrode was made the anode of the cell, wasemployed to anodically corrode a nickel/copper sulphur matte. Theparticulate anode comprised particles of crushed matte of sizes in therange of from 500 to 1650 μm. An electrolyte consisting of coppersulphate and sulphuric acid was flowed through the particulate anode.Current was fed to the particulate anode by a platinum sheet currentconductor of area 30 cm². A copper sheet was used as the cathode. Duringthe experiment an approximate current density of 300 A/m² was employed,this comparatively low current density being selected to ensure goodplating on to the planar copper cathode. Only traces of metal ionsdiffused through the ion permeable membrane so that the catholyte wasprogressively depleted of copper ions. Cathode current efficiency was 95%.

The matte dissolved faster than the equivalent current passed indicatingthe occurrence of chemical dissolution of the matte. Anode currentefficiency was 145 % based on the number of coulombs passed.

During the experiment, the matte rapidly disintegrated. On average, onethird of the weight of metal present in the matte was dissolved beforethe particles of the matte became too small to facilitate goodelectrical contact. The slude produced by the fine particles was carriedout of the cell.

We claim:
 1. In an electrochemical process wherein an electrolyte ispassed through one electrode compartment of an electrochemical cell,said one electrode compartment containing a particulate electrode whichcomprises a mass of discrete electroconductive particles and beingseparated from a second electrode compartment of said electrochemicalcell containing a counter-electrode by a separator having anion-permeable wall, the improvement which comprises (a) controlling thedistribution of the particles of said particulate electrode in theelectrolyte during the process in a manner such that there are presentwithin said mass a first region which is adjacent to the ion-permeablewall and within which first region the average number of particles perunit volume is relatively small so that at any given time substantiallyall of the particles are separated from each other, and a second regionwhich is spaced from the ion-permeable wall and within which secondregion the average number of particles per unit volume is relativelygreat so that at any given time substantially all the particles are incontact with other particles, and (b) establishing a circulation patternin said one electrode compartment such that particles circulate alongpaths passing through said first and second regions.
 2. Anelectrochemical process according to claim 1, wherein the distributionof the particles of said particulate electrode is controlled in such amanner that there is a transitional region disposed between, and havingan upright diffuse interface with, each of said first and secondregions, in which transitional region the number of particles presentper unit volume is intermediate the number of particles present per unitvolume of said first region and the number of particles present per unitvolume of said second region.
 3. An electrochemical process according toclaim 1, wherein most of the particles are caused to circulate in amanner such that they flow upwardly through a major proportion of theheight of said first region and downwardly through a major proportion ofthe height of said second region.
 4. An electrochemical processaccording to claim 1, wherein the distribution of the particles iscontrolled at least in part by introducing a fluid medium into a lowerportion of the particulate electrode and flowing said fluid mediumupwardly adjacent the ion-permeable wall.
 5. An electrochemical processaccording to claim 4, wherein said fluid medium is urged to flowupwardly at a rate sufficient to lift substantially all the particles ofsaid particulate electrode.
 6. An electrochemical process according toclaim 4 wherein said fluid medium comprises a liquid electrolyte.
 7. Anelectrochemical process according to claim 4 wherein said fluid mediumis an aqueous electrolyte.
 8. An electrochemical process according toclaim 1, wherein the volume, within an electrode compartment containinga particulate electrode, which is occupied by the particles during theprocess is less than 20% greater than the volume that would be occupiedby a static, settled bed of said particles.
 9. An electrochemicalprocess according to claim 8 wherein the volume occupied by saidparticles is in the range of from 8% to 12% greater than that occupiedby a static, settled bed of said particles.
 10. In a process forelectrowinning a metal wherein an electrolyte comprising an aqueoussolution of one or more salts of said metal is passed through a cathodecompartment of an electrochemical cell, said cathode compartmentcontains a particulate cathode which comprises a mass of discrete,electroconductive particles and being separated from an anodecompartment by a separator having an ion-permeable wall, the improvementwhich comprises (a) introducing small particles into said cathodecompartment wherein they form part of the particulate cathode (b)extracting particles on which metal has been electro-deposited from saidcathode compartment, (c) controlling the distribution of the particlesof said particulate cathode in the electrolyte during the process in amanner such that there are present within said mass a first region whichis adjacent to the ion-permeable wall and within which first region theaverage number of particles per unit volume is relatively small so thatat any given time substantially all of the particles are separated fromeach other, and a second region which is spaced from the ion-permeablewall and within which second region the average number of particles perunit volume is relatively great so that at any given time substantillyall the particles are in contact with other particles, and (b)establishing a circulation pattern in said one electrode compartmentsuch that particles circulate along paths passing through said first andsecond regions.
 11. A process according to claim 10, wherein thedistribution of the particles of said particulate cathode is controlledin such a manner that there is a transitional region disposed between,and having an upright diffuse interface with, each of said first andsecond regions, in which transitional region the number of particlespresent per unit volume is intermediate the number of particles presentper unit volume of said first region and the number of particles presentper unit volume of said second region.
 12. A process according to claim10, wherein most of the particles are caused to circulate in a mannersuch that they flow upwardly through a major proportion of the height ofsaid first region and downwardly through a major proportion of theheight of said second region.
 13. A process according to claim 10,wherein the distribution of the particles is controlled at least in partby introducing the electrolyte into a lower portion of the particulatecathode and flowing said electrolyte upwardly adjacent the ion-permeablewall.
 14. A process according to claim 10, wherein the volume which isoccupied by the particles of the particulate cathode during the processis less than 20% greater than the volume that would be occupied by astatic, settled bed of said particles.
 15. A process according to claim10 wherein the process comprises electrodeposition of at least onemember of the group of ions comprising the platinum group metals,copper, cobalt, nickel, zinc, manganese, silver, gold and iron.
 16. Aprocess according to claim 10 wherein the particulate cathodesubstantially consists of particles having a maximum dimension in therange of from 100 μm to 3000 μm.